Very low temperature single stage refrigeration system

ABSTRACT

A refrigeration system and method of operating a refrigeration system having a loop comprising a compressor, a condenser, an expansion device and an evaporator, with the compressor compressing a refrigerant gas thereby heating the gas to a hot gas and the condenser removing heat from the hot gas thereby transforming the hot gas to a liquid refrigerant. The compressor has an inlet receiving gas from the evaporator and an outlet supplying hot gas to the condenser. The expansion device expands the liquid refrigerant from the condenser to a liquid-gas in the evaporator thereby absorbing heat. A heat exchanger transfers heat from the liquid refrigerant supplied to the expansion device. This increases energy density of the refrigerant.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional Application, Ser. No. 62/841,985, filed May 2, 2019, which is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

The present invention is directed to a refrigeration system and method of operating a refrigeration system that is capable of obtaining very low temperatures. While the invention is particularly useful for cooling an environment test chamber it has other applications.

In order to obtain very low temperatures it is common to utilize a cascading refrigeration system of the type disclosed in commonly assigned US patent application publication US 2018/0320933 the disclosure of which is hereby incorporated herein by reference. Such cascading refrigeration system utilizes two refrigeration stages, thus requiring two compressors and associated hardware and controls. It is known to utilize a single stage refrigeration system having a single scroll compressor to achieve low temperatures as is disclosed in commonly assigned U.S. Pat. No. 6,374,621 the disclosure of which is hereby incorporated herein by reference.

SUMMARY OF THE INVENTION

The present invention is directed to a refrigeration system and method of operating a refrigeration system that is capable of obtaining very low temperatures. While illustrated with a single stage refrigeration system utilizing a scroll compressor it is capable of use with other types of compressors having the capability for intermediate pressure injection. It could also be used with a cascading refrigeration system.

A refrigeration system and method of operating a refrigeration system having a loop comprising a compressor, a condenser, an expansion device and an evaporator, with the compressor compressing a refrigerant gas thereby heating the gas to a hot gas and the condenser removing heat from the hot gas thereby transforming the hot gas to a liquid refrigerant. The compressor has an inlet receiving gas from the evaporator and an outlet supplying hot gas to the condenser. The expansion device expands the liquid refrigerant from the condenser to a liquid-gas mixture in the evaporator thereby absorbing heat. A heat exchanger transfers heat from the liquid refrigerant supplied to the expansion device. This increases the energy density of the refrigerant.

The compressor may have an additional inlet that can be used to inject refrigerant into the compression system at a pressure level between the evaporation and the condensation pressure. The loop may include an intermediate circuit that also cools the liquid refrigerant from the condenser and the heat exchanger transfers the heat from the liquid refrigerant supplied to the expansion device to the intermediate circuit refrigerant. The intermediate circuit may cool the refrigerant from the condenser with another expansion device. The other expansion device may expand refrigerant into the secondary coil of the heat exchanger of the intermediate circuit.

The compressor may be a compound compressor such as a scroll compressor having an intermediate pressure injection port and the intermediate refrigerant branch from the condenser may supply refrigerant to the intermediate pressure injection port via the other expansion device. The heat exchanger may have a primary coil connected between the condenser and the expansion device and a secondary coil that is connected between the other expansion device and the intermediate injection port. The refrigeration system may be capable of cooling from a temperature greater than room ambient to minus 50 degrees Celsius (−50° C.) and even capable of cooling to minus 70 degrees Celsius (−70° C.) or colder. The transferring of heat from the liquid refrigerant supplied to the expansion device may be effected without substantially increasing the suction pressure of the compressor.

These and other objects, advantages, and features of this invention will become apparent upon review of the following specification in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a refrigeration system according to an embodiment of the invention;

FIG. 2 is a pressure versus enthalpy diagram of the refrigeration system in FIG. 1; and

FIG. 3 is a schematic diagram of a detailed embodiment of the refrigeration system in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described with reference to the accompanying figures, wherein the numbered elements in the following written description correspond to like-numbered elements in the figures.

Referring now to the drawings and the illustrative embodiments depicted therein, a refrigeration system 10 includes a loop 12 having a compressor 14, a condenser 16, an expansion device 18 and an evaporator 20. Compressor 14 has an inlet that receives refrigerant from evaporator 20 and compresses the refrigerant. Due to inner friction the refrigerant gas heats up during compression, resulting in a higher discharge temperature which is compressor and refrigerant specific (hot-gas temperature). This is represented as the sloping line from 1 to 2 on the pressure/enthalpy diagram 11 in FIG. 2. Compressor 14 has an outlet at 2 supplying hot gas to condenser 16. Condenser 16 removes heat from the hot refrigerant gas thereby transforming the hot gas to a liquid refrigerant at 5. This is represented by the generally horizontal line between points 2 and 5 on diagram 11. It should be understood that compressor 14 is driven by a motor (not shown) and a fan 17 is provided to pass air over condenser 16 to remove heat therefrom. If the condenser is water or other fluidcooled then no fan is used.

The liquid refrigerant from condenser 16 passes through the primary coil 28 of a heat exchanger 22 where density of the liquid refrigerant is increased by the removal of heat from the refrigerant as indicated between points 5 and 3 on diagram 11. The refrigerant at 3 is then expanded by expansion device 18 in evaporator 20, thereby absorbing heat, as represented by the generally constant enthalpy line 3 to 4 in diagram 11. The refrigerant becomes a liquid-gas mixture in evaporator 20 and is returned to compressor 14 at a lower pressure. This cools the space (not shown) in which evaporator 20 is positioned such as in the interior of an environmental test chamber of the type disclosed in the '621 patent or other refrigeration application.

Refrigeration system 10 further includes an intermediate circuit 24 that also receives refrigerant in liquid form as indicated at 5 in diagram 11 and cools liquid refrigerant from condenser 16 as represented by the dashed constant enthalpy line 5 to 6 on diagram 11 by another expansion device 30 that expands the liquid in secondary coil 26 of heat exchanger 22. While expansion device 30 could be a capillary tube, in the illustrated embodiment it is a thermal expansion valve or an electronically controlled expansion valve that is controlled in response to the discharge/hotgas temperature of compressor 14.

This transfers heat from primary coil 28 of heat exchanger 22 thus removing heat from the liquid refrigerant supplied to expansion device 18 to further cool the refrigerant gas in the evaporator 20 to a sub-cooled condition. Heat is transferred from primary coil 28 to secondary coil 26 in intermediate circuit 24. Refrigerant is converted to a vapor state when it exits heat exchanger secondary coil 26 and is supplied via intermediate circuit 24 to an intermediate port at 7 of compressor 14 as represented by the generally constant pressure line 6 to 7 in diagram 11. In the illustrated embodiment compound or two-stage compressor 14 is a scroll compressor of the type marketed by Copeland Corporation but other types of compound or two-stage compressors could be used. Alternatively, the output of intermediate circuit 24 introduced at other portions of loop 12. The refrigerant supplied by intermediate circuit 24 to intermediate port at 7 of compressor 14 further cools the gaseous refrigerant output by compressor 14 as shown by the sloping line from points 7 to 2 in diagram 11. The refrigerant supplied to the intermediate port of compressor 14 may also be used to keep the compressor from over-heating. Various designs of such primary and secondary coils of heat exchanger 22 are known in the art.

In the illustrated embodiment (FIG. 3), expansion device 18 is a controlled expansion valve which is controlled in response to a temperature sensor 40 and pressure sensor 42 both sensing at the output of evaporator 20. A solenoid operated valve 44 is operated by the system control (not shown) to regulate the temperature produced by system 10. A temperature sensor 46 mounted to the case of compressor 14 controls expansion device 30 which is controllable in order to control the amount of refrigerant supplied to compressor 14 such as, for example, supply enough refrigerant to keeps compressor 14 from over-heating or providing excessive discharge temperatures. In the illustrated embodiment, expansion device 30 is a thermal expansion valve or an electronically controlled expansion valve.

Refrigerant R-410A is used in the illustrated embodiment although other refrigerants can be used. By increasing density of the refrigerant supplied to the primary evaporator the quality of the liquid refrigerant improves its low temperature performance. By reducing the liquid injection temperature to the intermediate port of the compressor the refrigeration system will gain efficiency. This should allow refrigeration system 10 to achieve temperatures of minus 50 degrees Celsius and possibly as low as minus 70 degrees Celsius or even lower. This can be achieved by operating the compressor on two different pressure levels. Using the intermediate injection (30) to maintain enough refrigerant mass flow through the compressor in order to keep the discharge temperature within an acceptable limit, the pressure in the evaporator (20) can be lowered below ambient pressure in order to lower the evaporation temperature. Utilizing a combination of sub-cooled refrigerant (temperature 5 to temperature 3 FIG. 2) and operating one compressor at two different pressure levels allows very low temperatures to be achieved with refrigerants that would not be possible without the disclosed embodiments. The regular operation range of the compressor may otherwise be exceeded resulting in possible damages on the compression system (e.g. due to the lack of lubrication or refrigerant mass flow resulting in excessive discharge temperatures)

The transfer of heat from the refrigerant supplied to expansion device 18 by heat exchanger 22 increases the energy density of the refrigerant. This increase in energy density is accomplished without requiring substantial altering of suction pressure of the compressor 14. This improves mass flow management of the refrigerant in loop 12.

While the foregoing description describes several embodiments of the present invention, it will be understood by those skilled in the art that variations and modifications to these embodiments may be made without departing from the spirit and scope of the invention, as defined in the claims below. The present invention encompasses all combinations of various embodiments or aspects of the invention described herein. It is understood that any and all embodiments of the present invention may be taken in conjunction with any other embodiment to describe additional embodiments of the present invention. Furthermore, any elements of an embodiment may be combined with any and all other elements of any of the embodiments to describe additional embodiments 

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A refrigeration system, comprising: a loop comprising a compressor, a condenser, an expansion device and an evaporator; said compressor compressing a refrigerant gas thereby heating the gas to a hot gas and said condenser removing heat from the hot gas thereby transforming the hot gas to a liquid refrigerant, said compressor having an inlet receiving gas from the evaporator and an outlet supplying hot gas to the condenser; said expansion device expanding the liquid refrigerant received from the condenser to a liquid-gas mixture in the evaporator thereby absorbing heat; and a heat exchanger transferring heat from the liquid refrigerant supplied to said expansion device thereby increasing energy density of the refrigerant supplied to said evaporator.
 2. The refrigeration system as claimed in claim 1 wherein said loop comprises an intermediate circuit that also cools liquid refrigerant from said condenser, said heat exchanger transferring heat from the liquid refrigerant supplied to the expansion device to the intermediate circuit refrigerant.
 3. The refrigerant system as claimed in claim 2 wherein said intermediate branch cools liquid refrigerant from said condenser with another expansion device.
 4. The refrigerant system as claimed in claim 3 wherein said expansion device comprises a thermally controlled expansion valve or an electronically controlled expansion valve.
 5. A refrigeration system, comprising: a loop comprising a compressor, a condenser, an expansion device and an evaporator; said compressor compressing a refrigerant gas thereby heating the gas to a hot gas and said condenser removing heat from the hot gas thereby transforming the hot gas to a liquid refrigerant, said compressor having an inlet receiving gas from the evaporator and an outlet supplying hot gas to the condenser; said expansion device expanding the liquid refrigerant received from the condenser to a liquid-gas mixture in the evaporator thereby absorbing heat; a heat exchanger transferring heat from the liquid refrigerant supplied to said expansion device to an intermediate refrigerant circuit; and wherein said compressor comprising a two-stage compressor having an intermediate injection port and said intermediate refrigerant circuit supplies refrigerant to the intermediate injection port.
 6. The refrigeration system as claimed in claim 5 wherein said two-stage compressor comprises a scroll compressor.
 7. The refrigeration system as claimed in claim 5 wherein said intermediate circuit also cools liquid refrigerant from said condenser with another expansion device wherein said heat exchanger comprises a primary coil connected between said condenser and said expansion device and a secondary coil that is connected between said another expansion device and said intermediate injection port.
 8. The refrigeration system as claimed in claim 7 wherein said another expansion device expands refrigerant in said heat exchanger secondary coil.
 9. The refrigeration system as claimed in claim 5 wherein said heat exchanger comprises a primary coil connected between said condenser and said expansion device and a secondary coil that is connected between said another expansion device and said intermediate injection port.
 10. The refrigeration system as claimed in claim 9 wherein said another expansion device expands refrigerant in said heat exchanger secondary coil.
 13. The refrigeration system as claimed in claim 1 that is capable of cooling to minus 50 degrees Celsius (−50° C.).
 14. The refrigeration system as claimed in claim 1 that is capable of cooling to minus 70 degrees Celsius (−70° C.).
 15. The refrigeration system as claimed in claim 1 wherein the transferring of heat from the liquid refrigerant supplied to said expansion device without requiring substantial change in suction pressure at said compressor inlet.
 13. The refrigeration system as claimed in claim 5 that is capable of cooling to minus 50 degrees Celsius (−50° C.).
 14. The refrigeration system as claimed in claim 5 that is capable of cooling to minus 70 degrees Celsius (−70° C.).
 15. The refrigeration system as claimed in claim 5 wherein the transferring of heat from the liquid refrigerant supplied to said expansion device without requiring substantial change to suction pressure at said compressor inlet.
 16. A method of operating a refrigeration system having a loop comprising a compressor, a condenser, an expansion device and an evaporator, said compressor compressing a refrigerant gas thereby heating the gas to a hot gas, said condenser removing heat from the hot gas thereby transforming the hot gas to a liquid refrigerant, said compressor having an inlet receiving gas from the evaporator and an outlet supplying hot gas to the condenser; said expansion device expanding the liquid refrigerant received from the condenser to a liquid-gas mixture in the evaporator thereby absorbing heat with the evaporator, said method comprising: transferring heat from the liquid refrigerant supplied to said expansion device to increase energy density of the refrigerant supplied to said evaporator.
 17. A method of operating a refrigeration system, having a loop comprising a compressor, a condenser, an expansion device and an evaporator; wherein said compressor compressing a refrigerant gas thereby heating the gas to a hot gas and said condenser removing heat from the hot gas thereby transforming the hot gas to a liquid refrigerant, said compressor having an inlet receiving gas from the evaporator and an outlet supplying hot gas to the condenser; and said expansion device expanding the liquid refrigerant received from the condenser to a liquid-gas mixture in the evaporator thereby absorbing heat; said method comprising: transferring heat from the liquid refrigerant supplied to said expansion device to an intermediate refrigerant circuit thereby lowering evaporator pressure below ambient pressure in order to further lower the evaporation temperature; wherein said compressor comprising a two-stage compressor having an intermediate injection port including supplying refrigerant from said intermediate refrigerant circuit to the intermediate injection port; and regulating the refrigerant from the intermediate circuit to the intermediate injection port in order to provide sufficient refrigerant mass flow through the compressor in order to keep the discharge temperature within an acceptable range; wherein the transferring of heat from the liquid refrigerant supplied to said expansion device and the supplying of refrigerant to the intermediate injection port achieves lower temperatures than would occur without said transferring and said supplying. 